The present invention relates to a method for manufacturing an electronic circuit board using a lead-free solder alloy having little toxicity, an apparatus therefor, and a soldered assembly using the same. The lead-free solder alloy is an alternative to Sn-37Pb (unit: mass %) solder used for soldering at around 220° C. which can be applied to connection of electronic component parts to a circuit substrate such as an organic substrate.
General lead-free solders constituted of Sn, Ag, Cu, Bi, Zn or the like are inferior in wettability to copper electrodes on a circuit substrate, as compared with the conventional Sn—Pb based lead-containing solders, and the lead-free solders are known to be strongly affected by oxidation of electrodes due to oxygen present in the atmospheric air.
It is known that the poor wettability has a great influence on the easiness of flowing-in of solder in the penetrating holes accompanied by copper electrode called through-holes for fixing inserted component parts to a substrate. For example, when the wettability becomes bad, the amount of the solder flowing into the through-hole is reduced, and the solder would not comes up to the upper side of the substrate through the through-hole, resulting in that the flow-up ratio is lowered. The flow-up ratio is defined by the value obtained by dividing the length from the lower side of the substrate to the level to which the solder comes up by the length from the lower side to the upper side of the substrate. When the flow-up ratio is thus lowered, the leads of the inserted component parts are fixed with small amounts of solder, resulting in that the reliability of the connection portions is lowered.
It is said that a flow-up ratio of about 80% is sufficient for contriving high reliability of connection portions. Therefore, in a wave soldering apparatus applicable to lead-free solder, soldering must be performed in an inert atmosphere such as nitrogen with an oxygen concentration of generally not more than 1%, and the soldering portion must be located in an inert gas atmosphere chamber for filling with an inert gas such as nitrogen.
In addition, it is known that when wave soldering (flow soldering) of inserted component parts is conducted by the use of lead-free solder, the so-called lift-off phenomenon of exfoliation of the solder at the connection portion from the copper electrode on the substrate occurs. In order to restrain the lift-off phenomenon, the substrate must be rapidly cooled immediately after soldering, so that a large amount of a refrigerant must be blown to the soldered portion. Besides, since the soldering is conducted in an inert atmosphere such as nitrogen, the refrigerant is limited to the same fluid as the inert atmosphere.
In addition, in the apparatus for conducting wave soldering of inserted component parts by the use of lead-free solder, there has recently been a demand for frequent changes of the wave soldering conditions so as to allow the apparatus to be applicable to many kinds of substrates.